The electrical conductivity of a liquid is an important analysis parameter of electrochemistry. Its measurement has a wide application in fields like the chemical industry, metallurgy, biology, medicine, grain testing, water conservancy, energy resources, etc. Conductivity measuring methods can be divided into 2 groups: contact-type and non-contact type.
A non-contact type measurement applies the principle of electromagnetic induction and is therefore also referred to as an electromagnetic conductivity-measuring method or an inductive conductivity-measuring method. As there is no contact between the conductive part of the measuring component and the measured liquid, sensors of this type possess the advantages of good solidity, corrosion resistance, non-polarization and long service life. There has been a long history of development since the basic principle of electromagnetic measurement of the conductivity of a liquid was invented and applied in practice.
For example, U.S. Pat. No. 2,542,057 to M. J. Relis opened the basic theory to the public in 1951. The sensor according to this reference employs a pair of coaxial toroidal cores which are covered by corrosion-protective and electrically insulating material. The inner hole of the 2 toroidal cores allows the current path through the liquid. According to the electromagnetic induction principle, when an alternating current is sent through the excitation coil, an alternating magnetic flux is generated in the exciting toroidal core, which in turn generates an induction current through the loop in the measured liquid. The induction current generated in the loop presents itself as a current loop which crosses both the exciting toroidal core and the pick-up toroidal core. This current loop generates an alternating magnetic flux in the toroidal core, which generates in the induction coil an induced current, which in turn produces an induced electrical voltage at the induction coil.
Because the induction current of the liquid is related to its conductivity, the induced current and the induced voltage of the induction coil (open-circuit voltage) is proportional to the current through the liquid. Thus, the conductivity of the liquid can be derived from the measurement of the induced current or the induced voltage. The conductivity G of the liquid is calculated from the formula G=C/R, wherein C is the sensor cell constant and R is the equivalent resistance of the loop through the liquid. In the past, the excitation voltage was usually an AC sine-wave, and the induced voltage of the induction coil was measured by an electric bridge-balancing method, which had the disadvantages of low precision and a low level of automation. At present, due to the development of modern electronic technologies, this method is rarely used.
The method of measuring the induction voltage is relatively simple and is still being used. For example, according to the method which was introduced in the publication “Inductive Conductivity and Concentration Meter”, Chemical Automation and Meters, 1997, 24(1): 56-58, the induction current of the liquid is related to its conductivity. The induced current or the induced voltage (open-circuit voltage) of the induction coil is proportional to the current through the liquid. Hence, the conductivity of the liquid can be derived from the measurement of the induced current or the induced voltage of the induction coil. But in this method, the induced voltage of induction coil is not only related to the conductivity of the liquid, but also to the inductance of the excitation coil, which negatively affects the linearity of the measurement. Also, the magnetic permeability of the toroidal core is affected by temperature and other factors, which causes a temperature-dependent drift of the inductance of the excitation coil and has a negative effect on the precision of the measurement.
To increase the accuracy of the measurements U.S. Pat. No. 5,455,513 A1 to Neil L. Brown proposes a system, which employs a current-compensation method, also known as zero-current method. Thereby the induced current of the induction coil is balanced by an additional compensation such that the compensation current is subtracted from the induction current to produce a zero-current and a corresponding zero-voltage. This is a method of relatively high precision, because when the voltage at the measurement terminal of the induction coil is zero, the induced current in the induction coil is proportional to the conductivity of the liquid. However, this method is relatively complicated and costly, because it involves the steps of pre-amplification, tuned filter amplification, in-phase detection, integration, switching multiplication and further amplification to generate the appropriate compensation current. Further, to change the measurement range, it is usually necessary to change the parameters of all the involved components. Also for the integration step mentioned above, a high quality integration capacitor is required, and therefore the cost is high.